The DS-CDMA communication method is a scheme that transmits information data after spreading their bandwidth using a code with a rate much higher than the information data rate, and its research and development have been intensively conducted to be applied to cellular systems. This is because the DS-CDMA systems have such characteristics as facilitating flexible cell design which will enable the capacity in terms of the number of users to be increased as compared with the conventional frequency division multiple access (FDMA) or time division multiple access (TDMA).
The DS-CDMA systems include two spreading methods: One carries out spreading using a spreading code called a "short code" with a period equal to that of the information symbols; and the other carries out spreading using a spreading code called a "long code" with a period much longer than that of the information symbols. As the spreading codes, Gold codes or others are used. The Gold codes consists of two M (maximum) sequences, and the Gold codes belonging to the same group can be generated by the number corresponding to its period.
Accordingly, the number of the Gold codes that can be generated is no more than that corresponding to the processing gain (PG) or spreading ratio. As a result, in the cellular systems, the same spreading code cannot be used within several cells because of interference from other cells, and this presents a reuse problem in spreading code assignment.
On the other hand, using a long code enables to generate a great number of codes by lengthening its period. Thus, each cell can assign spreading codes to users independently of the other cells in the multicellular configuration. This is because the probability is very small that the same code is used at the same time in another cell thanks to the great number of the codes.
In the cellular systems, besides the incoming radio wave traveling through the shortest path from the transmitting point, there are delayed waves resulting from reflection and refraction due to obstacles or configuration of ground such as surrounding buildings, mountains, towers, etc. Since the delayed waves usually become interference signals against desired waves, they will degrade received characteristics. In the DS-CDMA system, the information signals are transmitted as very fast signals. Thus, when they are spread to 1 MHz band, the desired waves can be separated from delayed waves with a delay of one microsecond by carrying out correlation detection at a resolution of one microsecond. Combining these waves after demodulating independently, which is called RAKE combining, has an advantage of making full use of the power of the delayed waves.
In this case, since each one of consecutive information symbols is spread by a spreading code of the same pattern in the short code system, the delayed waves with a delay beyond one information symbol cannot be combined. On the contrary, since the consecutive information symbols are spread with different portions of a long code in the long code system, the delayed waves with a delay beyond one information symbol can be RAKE combined.
Although the long code has various merits as described above, it has a demerit that it takes a long time to establish the synchronization of the spreading code. Specifically, a DS-CDMA receiver must establish synchronization of the phase of a spreading code replica at the receiver side with that of the spreading code in a received signal at the beginning of communications. Since the long code has a much longer spreading code phase to be searched for compared with the short code, much longer time is required for establishing the synchronization.
The receiver conducts the correlation detection using a matched filter as shown in FIG. 3 and a sliding correlator as shown in FIG. 4.
(Description based on FIG. 3)
The matched filter usually includes delay elements 1 with a delay of one chip, and spreading code multipliers 2, the number of each of them corresponds to the processing gain. Each of the spreading code multipliers 2 can be composed of an exclusive OR (EXOR) circuit because the spreading code replica is usually binary. A spread modulation signal which has been frequency converted to the baseband with the zero IF frequency and is input to the matched filter, is delayed by the number of times of the processing gain, and they are each multiplied by a spreading code replica fed from a spreading code replica generator 3. The resultant product signals are summed up by an adder 4. When the phase of the spreading code in the spread modulation signal is synchronized with that of the spreading code replica, the output of the adder 4 takes a peak correlation value whose power is increased by a factor of processing gain from the average power obtained with asynchronous phases. Thus, since the matched filter detects the correlation using space integration, it has an advantage of shortening the initial acquisition time of the spreading code.
(Description based on FIG. 4)
In the sliding correlator, a multiplier 6 multiplies the spread modulation signal by a spreading code replica generated by a spreading code replica generator 5, and then an integrating/dumping circuit 7 performs integral of the resultant product by an amount of the processing gain. The multiplier can be composed of an EXOR because the spreading code replica is usually binary. The integral time in the integrating/dumping circuit 7 is usually one information symbol period. The integrated signal is square-law detected by a square-law detector 8 to generate its amplitude component whose value undergoes threshold decision by a threshold value decision circuit 9. Thus, the a decision is made whether they are synchronized or not. If the integral value does not exceed the threshold value, a decision is made that they are not yet synchronized, and the threshold value decision circuit 9 controls a digitally controlled clock generator 10 such that the phase of the spreading code replica output from the spreading code replica generator 5 is updated by advancing it by J chips (usually, J=1). Thus, since the sliding correlator performs time integral, it is smaller than the matched filter in circuit scale, but takes a longer time for the initial acquisition.
As described above, the matched filter requires a shorter acquisition time thanks to the space integral, but is larger in the circuit scale. On the contrary, the sliding correlator is smaller in the circuit scale thanks to the time integral, but requires a longer acquisition time.
Defining that
A: the number of long codes to be searched, PA1 Q: the number of phases of the entire chips of a long code to be searched, PA1 PG: the processing gain, PA1 M: the number of symbols to be integrated for correlation detection, PA1 Tc: chip period, PA1 NSC: the number of sliding correlators, PA1 NMF: the number of matched filters PA1 TSC: acquisition time of the sliding correlators, and PA1 TMF: acquisition time of the matched filters, the acquisition times of the sliding correlator and the matched filter become as follows when there are no thermal noise, or no cross-correlation due to interference from the delayed waves from other users or its own channel signal. PA1 using a first spreading code group and a second spreading code group, the first spreading code group being common to respective base stations and having a period equal to an information symbol period, and the second spreading code group being different from base station to base station and having a period longer than the information symbol period; and PA1 masking, when transmitting a signal which is doubly spread using a first spreading code in the first spreading code group and a second spreading code in the second spreading code group, the second spreading code for M symbols at fixed intervals, where M is a natural number equal to or greater than one. PA1 first synchronization detecting means for detecting a synchronized time of a first spreading code from a detection time of a correlation output value, the correlation output value being obtained through a correlation detection processing between a spread modulation signal obtained by receiving a signal transmitted by the transmission means of claim 1 and a first spreading code in the first spreading code group of claim 1; and PA1 second synchronization detection means for performing correlation detection sequentially using codes obtained by multiplying the first spreading code by A (A is a natural number) second spreading codes in the second spreading code group of claim 1, and for deciding a second spreading code having a maximum correlation value, wherein the correlation detection is started from a time position at which a maximum correlation value is detected by the first synchronization detection means. PA1 first spreading code synchronized phase memory means for storing B dominant time positions in descending order of magnitude of correlation values detected by the first synchronization means of claim PA1 second spreading code synchronization detecting means for performing correlation operations sequentially between a received signal and codes obtained by multiplying the first spreading code of claim 1 by B spreading codes in the second spreading codes of claim 1 of contiguous base stations of a current base station of which the current base station notifies, wherein the correlation operations are started from time positions stored in the first spreading code synchronization memory means, and are carried out in descending order of magnitude of the correlation values stored in the first spreading code synchronization memory means; and PA1 means for detecting which codes of the second spreading codes correspond to the B dominant time positions of the first spreading codes of claim 1. PA1 performing correlation detection between a received spread modulation signal and codes obtained by multiplying the first spreading code of claim 1 by second spreading codes of the second spreading code group of claim 1 to decide the second spreading code used for spreading the received spread modulation signal, wherein the correlation detection is started from a time position at which a maximum correlation output signal is obtained in correlation detection between the first spreading code of claim 1 and the received spread modulation signal obtained by receiving a signal transmitted by the transmission method of claim 1; and PA1 deciding, after carrying out the correlation detection between the received spread modulation signal and the codes obtained by multiplying the first spreading code by the second spreading codes, the second spreading code giving a maximum correlation value as the second spreading code used for spreading the received spread modulation signal. PA1 the first spreading code synchronized phase memory means of claim 4; and PA1 received level detection means for detecting received signal power by generating delay profiles of multipaths for each base stations by detecting correlations between a received spread modulation signal and codes obtained by multiplying a first spreading code by second spreading codes of a current base station and contiguous base stations in a particular time range around a time position of the first spreading code synchronized phase memory means, PA1 wherein the received level detecting means carries out, in a second and following searches, a searching around a time position of a path obtained by previous search. PA1 a first synchronization detection step of performing correlation detection processing between a spread modulation signal obtained by receiving a signal transmitted by a transmission method of claim 1 and a first spreading code of the first spreading code group of claim 1 to detect a synchronized time of the first spreading code from a detection time of the correlation output value; and PA1 a second synchronization detection step of performing correlation detection sequentially on codes obtained by multiplying the first spreading code by A (A is a natural number) second spreading codes in the second spreading code group of claim 1 to decide a second spreading code giving a maximum correlation value, wherein the correlation detection is started at a time position at which a maximum value is obtained which is detected by the first synchronization detection step. PA1 a first spreading code synchronized phase memorizing step of storing B dominant time positions in descending order of magnitude of correlation values detected by the first synchronization step of claim 10; PA1 a second spreading code synchronization detection step of performing, in descending order of magnitude of correlation values, correlation operations sequentially between a received signal and codes obtained by multiplying the first spreading code of claim 1 by B spreading codes of the second spreading codes of claim 1 of contiguous base stations of a current base station of which the current base station notifies, wherein the correlation operations are started from time positions stored in the first spreading code synchronized phase memorizing step; and PA1 a step of detecting which second spreading codes correspond to B dominant time portions of the correlation values with the first spreading code of claim 1. PA1 receiving a signal transmitted in the transmission method of claim 19; PA1 detecting a received timing of a second spreading code by detecting correlation between the received signal and a shared first spreading code; PA1 detecting a second spreading code group including a second spreading code to be used for spreading the received signal by detecting correlation between the received signal and first spreading codes in a first spreading code group at received timings of signals spread by unshared first spreading codes, which received timings are obtained from received timings of the second spreading codes, and by deciding the unshared first spreading code giving a maximum correlation; and PA1 identifying the second spreading code used for spreading the received signal from magnitudes of correlation values detected between the received signal and spreading codes obtained by multiplying the shared first spreading code by the second spreading codes in the second spreading code group detected in the preceding step. PA1 receiving a signal transmitted by the transmission method of claim 18; PA1 detecting received timings of signals spread by only the first spreading code from timings giving maximum correlation values obtained by observing correlation between the received signal and the first spreading code at every interval of the L/n periods; PA1 detecting n received timings of the second spreading code which are shifted by an amount of the L/n periods from one another using the received timings of the signals spread by only the first spreading code; and PA1 detecting correlation values between the received signal and the spreading codes obtained by multiplying the first spreading code by the second spreading codes in the second spreading code group at phases synchronized with the detected n received timings of the second spreading code shifted by the amount of L/n periods to identify the second spreading code to be used for spreading the received signal from magnitudes of the correlation values and to determine n received timing candidates of the second spreading code. PA1 first code spreading means for spreading signals of all channels using first spreading codes which belong to a first spreading code group and differ from one another, the first spreading code group being common to respective base stations and having a period equal to an information symbol period; PA1 second code spreading means for spreading for M symbols one or more spread signals fed from the first code spreading means using a third spreading code, where M is a natural number equal to or greater than one, the third spreading code being a complex conjugate of a second spreading code which differs from base station to base station and has a period longer than the information symbol period; PA1 adding means for adding at appropriate timings a signal on a channel spread by the first code spreading means and signals of one or more channels spread by the second code spreading means; and PA1 third code spreading means for spreading by using the second spreading code the signals of the channels output from the adding means.
In the case of the sliding correlator: EQU TSC=A.times.Q.times.PG.times.M.times.TC/NSC
In the case of the matched filter EQU TMF=A.times.Q.times.M.times.TC/NMF
When using the long code, since the number A of long code and the number Q of phases to be searched are enormous, there is a problem in that it takes a very long acquisition time.